In order to improve a performance of an ABS used in a vehicle, it is considered to be effective to control lock and unlock states in a condition where friction coefficient of a road surface be as large as possible. The friction coefficient of the road surface depends on a slip rate of a tire/wheel assembly under a certain condition of the road surface and therefore the ABS is designed to control lock and unlock states of braking near a slip rate providing the maximum friction coefficient of the road surface.
For this reason, it is a common practice for the conventional ABS to measure a speed of the vehicle and a revolution of the tire/wheel assembly, calculate the slip rate based on the measured values, and automatically control the braking so as the slip rate to fall within a certain range.
However, the method for estimating the frictional coefficient of the road surface from the slip rate has a problem in which the relationship between the slip rate and the friction coefficient of the road surface drastically changes depending on a road surface condition to vary the slip rate corresponding to the optimum friction coefficient of the road surface depending on the road surface condition, so that the optimum friction coefficient of the road surface cannot be obtained from the slip rate alone. Although approaches for solving this problem such as separately estimating the road surface condition as well has been made, satisfactory means have not been proposed yet.
Meanwhile, in connection with this point, an approach for more directly measuring and estimating the friction coefficient of the road surface. Such an approach is known from the disclosure of Japanese Patent Application Opened No. 06-288798A. According to this disclosure, a strain gauge is attached to a suspension suspending the tire/wheel assembly and a strain occurring on this is measured to give a component parallel to the friction force of the road surface or a component perpendicular to the former component of a force acting on the suspension. The means estimate the friction coefficient of the road surface based on these measured values by assuming the values as the friction force of the road surface and the vertical force, respectively.
However, although the method with using a strain gauge is a more direct estimating method as compared with the method of estimating the friction coefficient with the slip rate, there are problems in which the point of measuring the force is far from a vicinity of tire which is the actual point of action of the friction force, so that a measuring result having been influenced by disturbances applied between the point of action of the friction force and the point of measuring the force is obtained, and that the strain gauge is applied on the suspension in which a generated strain is small, and the generated strain is converted to the force, so that its accuracy is not sufficient.
The present invention has been made in view of these problems. It relates to a method for directly estimating friction coefficient of a road surface independently from the slip rate, and its object is to provide a method of real-timely and more accurately estimating the friction coefficient of the road surface by measuring the friction force of the road surface and the vertical force at a region near the tire as well as accurately measuring these forces, and a method and a device of multiplex transmission of signals upon transmitting a measurement signal of a force measured in the vicinity of tire to the ABS displaced on the vehicle body side.